Breakwater Crest Elevation Calculator

Set waves, slope, roughness, and water levels fast. See crest freeboard and runup instantly today. Download logs to share with engineers and clients securely.

Inputs
All values are referenced to your selected datum.
Reset
Conversions are handled automatically.
Example: “Chart Datum”, “MSL”, or “Site Benchmark”.
Water Levels
Enter elevations relative to your datum.
Tide + surge level for the design event.
Set to zero if unknown.
Choose per design horizon and policy.
Wave Conditions
Use near-structure values (at the toe) when available.
Random sea significant wave height.
Used for wavelength and surf similarity.
1.0 for head-on. Use 0.8–0.95 for oblique.
Structure
Define the seaward slope and surface behavior.
:
Example: 1:2 gives tan(α)=0.5.
Typical rock armor: 0.55–0.70. Smooth: closer to 1.0.
Use 1.0 unless you have calibrated data.
Allowances
Add project-specific margins and adjustments.
Extra height for overtopping performance targets.
Account for foundation and armor bedding.
Optional conservative margin for uncertainty.
Add if a parapet/crest wall is included.
After calculation, results will appear above this form.
Example Data
A sample scenario to show typical inputs and outputs.
SWL (m) Hm0 (m) Tm-1,0 (s) Slope (V:H) γ Setup (m) Allowances (m) Ru2% (m) Zcrest (m)
2.50 3.00 8.0 1:2 0.60 0.30 1.40 5.61 9.81
Allowances include freeboard, settlement, sea level rise, and safety margin.
Formula Used
Symbols are based on common coastal engineering practice for run-up estimates.
1) Deep-water wavelength
Lm-1,0 = g · (Tm-1,0)² / (2π)
2) Iribarren number (surf similarity)
ξ = tan(α) / √(Hm0 / Lm-1,0), tan(α)=V/H
3) Wave run-up level exceeded by 2% of waves
Ru2% / Hm0 = γ · γβ · (1.65 · ξ)
Ru2% / Hm0 ≤ A · γ · γβ · (4 − 1.5 / √ξ)
4) Crest freeboard and crest elevation
Rc = Setup + Ru2% + Freeboard + Settlement + SLR + Safety
Zcrest = SWL + Rc + CrestWall
Tip: If you have a target overtopping discharge, adjust “Added freeboard” to meet your criterion using your preferred overtopping guidance.
How to Use This Calculator
  1. Choose units and enter your reference datum label.
  2. Enter the design still water level, including tide and surge.
  3. Provide near-structure wave height and period for the event.
  4. Set the seaward slope and select a roughness factor.
  5. Add allowances for overtopping, settlement, and sea level rise.
  6. Press calculate to view crest elevation above the header.
  7. Use “Add to Log” to compare multiple design scenarios.
Quick Guidance
  • γ lowers run-up for rough, permeable armor.
  • γβ reduces run-up for oblique waves.
  • Use toe wave values when transformation is significant.
  • Include settlement and sea level rise for life-cycle design.
For critical infrastructure, confirm with detailed modelling and the project’s adopted standards.
Downloads
Exports are available after you calculate and log rows.

Crest Elevation Design Objective

Breakwater crest elevation controls overtopping, backshore flooding, and armor stability during the design event. Engineers typically target a defined mean overtopping discharge (q) or an acceptable frequency of spray and green water. This calculator builds a transparent crest level by combining still water level with freeboard components, so design intent is documented. It supports quick iterations when wave climate, levels, or safety margins change.

Water Levels and Reference Datum

Use a consistent datum (chart datum, MSL, or project benchmark) and enter the design still water level (SWL). SWL should include astronomical tide plus storm surge, and may include wave setup if it is treated as a level shift. For long-life assets, a sea-level allowance (for example 0.2–0.6 m over the design horizon) can be added explicitly.

Wave Climate Inputs Near the Structure

Crest design depends on wave conditions at the breakwater toe, not offshore values. Provide significant wave height (Hs) and period (Tp or Tm-1,0) after transformation through shoaling, refraction, and sheltering. If depth-induced breaking limits the toe waves, adopt the reduced Hs and note the governing water depth used for checks.

Run-up and Freeboard Components

Freeboard is split into a run-up allowance (Ru2% or another percentile), a safety margin, and any crest wall or parapet height. Roughness, slope angle, berms, and permeability reduce run-up; smooth impermeable slopes increase it. Many manuals relate Ru2% to Hs and the surf similarity parameter; calibrate with local guidance and physical or numerical modeling when warranted. For crown walls, include the wall height above crest and verify sliding and overturning under impulsive loads.

Checks, Uncertainty, and Documentation

Compare the resulting crest with functional requirements: access, utilities, navigation, and visual constraints. Add settlement and construction tolerance allowances where quarry stone placement or foundation consolidation is expected. Perform sensitivity runs by varying SWL, Hs, and run-up factors by ±10% to see which inputs drive crest elevation. Use the exported CSV/PDF to archive inputs, outputs, and assumptions for design reviews and future upgrades.

FAQs

1) What is the still water level (SWL) input?

SWL is the design water surface without waves, referenced to your chosen datum. Include tide and surge, and add setup only if you treat it as a level increase.

2) Which wave height should be used in the calculator?

Use significant wave height at the breakwater toe or near the structure. If you only have offshore data, transform it for depth and sheltering, and apply breaking limits where appropriate.

3) How do I select the run-up allowance (Ru2% or similar)?

Choose a run-up percentile that matches your overtopping objective and guidance used on your project. Use slope, roughness, berm, and permeability factors from your method, or confirm with modeling for exposed sites.

4) Why include a safety margin in freeboard?

Margins cover uncertainties in waves, levels, coefficients, and construction tolerances. They also provide resilience against future changes, such as sea-level rise or altered storm statistics.

5) When is a crest wall or parapet recommended?

Use a crest wall when overtopping must be reduced but raising the entire crest is impractical. Confirm structural stability under wave impact and ensure the wall height is added above the armor crest in your section.

6) Can I rely on this output as a final design crest elevation?

Treat the result as a transparent estimate to compare options and document assumptions. Final crest selection should follow your governing standards, include geotechnical settlement checks, and be reviewed alongside overtopping and stability analyses.

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